Scaling for Devices

So I'm working on a mobile app but I suppose this applies to all devices.
Right now I'm working on having text display messages (like a chat system) and I'm using my normal method for scaling but I'm thinking this might actually be a bad practice:

I have a variable:

Code:

line_height_percent = .05;

ok and so this means that the height of the text will be 5% of the screens height (no matter what the size of the screen is)
This means I can fit 20 lines of text on any screen size. And this is awesome for consistency so I know every phone can fit the same amount of lines.
However now I'm thinking what if someone has a smaller phone vs like maybe a tablet. Should the tablet really have such a bigger size than the small phone? Will this be bad for small phones who now will have small text and might not be able to read the text? Should I just have a constant font size / get the font stored in the phones settings?

If you think having a 5% font size is fine then I'd prefer to keep it that way so I don't have to deal with the dpi change across devices as well....